1. Field of the Invention
This invention relates to the installation of signal transmission cable.
More particularly, the present invention relates to method and apparatus of the type especially adapted for pulling an extended length of fiber optic cable through a subterranean duct.
In a further and more specific aspect, the instant invention concerns improvements in methods and apparatus according to the foregoing for monitoring and sensing selected characteristics of the cable during pulling and modulating the pulling accordingly.
2. Prior Art
Various types of cable especially adapted for the transmission of audio and visual signals are well known. Conventional cable, for example, incorporates metallic wire as the medium for transmission of signals in the form of electric current. More recently, the art has directed attention to fiber optic cable in which encoded light pulses are transmitted through thin fibers of glass, plastic or other transparent material.
Signal transmission cables are subject to varying installation schemes. Especially common are airborne installations and underground installations. In an airborne scheme, the cable is suspended between supporting members such as poles. In an underground system, the cable is buried. Certain types of cable are suitable for direct installation. Others require a protective encasement.
In an underground or subterranean system it is generally preferred that the cable resides within a conduit commonly termed a subduct. The subduct provides protection for the more fragile types of cable. The subduct also facilitates maintenance of the cable and may accommodate subsequent installation of additional cable.
Typically, a subterranean system includes a subduct of extended length, frequently many miles, which has been buried by conventional practice such as plowing, trenching and filling. Accessibility is provided by a series of manholes or vaults, each having an opening at ground level. Entrance and exit vaults reside at respective ends of the subduct. Access vaults are placed at spaced locations intermediate the entrance and exit vaults.
The several access vaults divide the subduct into plurality of segments. Each segment has an entrance or upstream end at one vault and an exit or downstream end at the subsequent vault. A pull line extends through the subduct. A slack length of the pull line, commonly incorporated into the subduct prior to burial, is available within each vault.
The signal transmission cable is pulled through the subduct by the pull line. The cable is supplied by a spool thereof placed near the entrance vault. The upstream end of the pull line and the cable, in sequence, are drawn through the subduct. The operation is completed when the downstream end of the transmission cable is received within the exit vault and the pull line is stowed, usually upon a take-up spool.
It is desirable that cable be installed in continuous runs of maximal length. Splicing is a laborious, expensive and time consuming task. Additionally, each splice adversely effects transmission quality. The foregoing is substantially more pronounced in fiber optic cable than in wire cable. Further limitations, restricting the length of a single pull, are the result of the inherent tensile weakness of fiber optic cable. Accordingly, the prior art has devised various techniques for pulling extended lengths of transmission cable, especially fiber optic cable.
In accordance with one scheme, the installation is accomplished by a sequence of pulls and stores. Initially, cable is pulled from a supply spool through the first segment of the subduct and stored at the first access vault. Subsequently, the cable is pulled through the second segment and stored at the second access vault. The process is repeated in sequence temporarily storing cable at each access vault and subsequently pulling the cable through the adjacent downstream segment.
More recently, the art has provided means and methods for simultaneously pulling a cable through at least two adjacent segments of a subduct. The pull line is wound about the capstan wheel of a winch placed in the exit vault and in each access vault. The several capstan wheels are simultaneously driven by individual hydraulic motors, each powered by hydraulic fluid of pre-set maximum pressure. Tension exerted by a downstream winch pulls the cable into frictional driving engagement with the rotating wheel of the adjacent upstream winch.
Although providing certain advantages over previous efforts, the multiple winch system described above is not considered to be a panacea. Initially it is noted that set-up is laborious and time consuming, requiring that each winch be lowered into a vault and frequently requiring partial disassembly and reassembly. Further, since the capstan wheel is driven by a motor which responds to a predetermined fluid pressure system it is possible that excessive tension can be exerted upon the cable. Also the system requires the constant observation of a workman for adjusting and controlling each winch. Other shortcomings are noted.
In an airborne scheme, the cable is suspended between supporting members such as poles. Typically, a support cable is already present, and the more delicate signal transmission cable is secured thereto. First, however, the transmission cable must be pulled along the support cable. Conventionally this has been accomplished by attaching one end of a pull line to an end of the transmission cable, and attaching the other end of the pull line to a pull vehicle such as a truck or the basket of a boom truck. The pull cable is generally run through pulleys coupled to the support cable. Additional pulleys are added as the cable is pulled off its storage spool. This is a very simple and effective method of stringing cable for short distances, but, as discussed above for subterranean cable work, it is desirable that cable be installed in continuous runs of maximal length. Splicing is a laborious, expensive and time consuming task. Additionally, each splice adversely effects transmission quality. The length of a single pull is limited by the inherent tensile weakness of transmission cable, and especially limited by fiber optic cable. When that limit is reached, the cable must be collected such as on a take up spool, and a new run begun. This can be a tedious, time consuming procedure, and very difficult to implement if a pull truck is used.
Another problem which arises from using a vehicle to pull a cable, is subjecting the cable to a tension force in excess of a recommended value. A large pull vehicle can greatly exceed the tensile strength of a fiber optics cable, damaging the cable before the operator is even aware there is a problem. This has been overcome to some extent by the use of tensometers coupled between the pull vehicle and a cable. Typically the tensometer is coupled to a display within the cab of the vehicle and shows the tension on the cable as it is pulled. When the tension reaches a predetermined level, the vehicle operator stops pulling. This works well when the operator is paying close attention to the display as he drives, and if he can stop the vehicle at any time along the route of the cable run. Generally, however, the operator cannot stop in certain spots, for example on railroad tracks, intersections or the like. Therefore, the operator must stop where he can, before he exceeds the recommended tension. Furthermore, if a snag develops while pulling the cable, the recommended tension can be greatly exceeded before the operator can react to the warnings from the display, even if he is paying close attention.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide improvements in the installation of signal transmission cable.
Another object of the invention is the provision of improvements especially adapted for the installation of fiber optic cable in a subduct.
And another object of the invention is to provide an improved cable pulling system which operates in response to actual cable tension.
Still another object of the instant invention is the provision of improved means whereby an upstream pulling means is actuated and regulated in response to a downstream pulling means.
Yet another object of the invention is to provide means for regulating the speed of operation of an upstream pulling means in response to the speed at which a cable is pulled by a downstream pulling means.
Yet still another object of this invention is the provision of means for monitoring the upstream stress upon a cable and maintaining the stress below a predetermined value.
And a further object of the invention is to provide a system which is self-regulating and does not require constant observation and control by a workman.
Still another object of the immediate invention is the provision of an integrated system including various ancillary functions such as lubricating the cable and taking-up the expended pull line which function at a rate proportional to a rate at which the cable is pulled.
Yet a further object of the invention is to provide a cable pulling system that can be emplaced and made operational with comparative convenience and ease.
And yet another object of the invention is the provision of improvements according to the above which are relatively unencumbered and inexpensively practiced.
And another object of the present invention is to provide an apparatus for use on pull vehicles, to prevent exceeding a predetermined tension placed on a cable being pulled.